Field of the Invention
One disclosed aspect of the embodiments relates to an electromechanical transducer and a method of producing the transducer. More specifically, one embodiment relates to an electromechanical transducer that is used as an ultrasonic transducer and a method of producing the transducer.
Description of the Related Art
A capacitive electromechanical transducer produced by micromachining technology has been being researched as a substitute for piezoelectric devices. In the capacitive electromechanical transducer, charging of an insulating film constituting an element changes the effective voltage applied between opposing electrodes, and thereby the conversion efficiency varies. The conversion efficiency herein is the efficiency of converting vibration of a vibration film into an electrical signal. The conversion efficiency is increased with an increase in the voltage applied or with a decrease in the distance between the electrodes. The variations among the cells or the elements in conversion efficiency cause variations in sensitivity and bandwidth of the electromechanical transducer.
Japanese Patent Laid-Open No. 2008-288813 describes an electromechanical transducer in which the charge storage (charging) of an insulating film disposed between electrodes is prevented and, at the same time, the pressure resistance of the insulating film between the upper and lower electrodes is improved. In the electromechanical transducer described in Japanese Patent Laid-Open No. 2008-288813, the insulating film being in contact with the lower electrode is a silicon oxide film, and the insulating film (membrane) being in contact with the upper electrode on the space side is also a silicon oxide film. Silicon oxide films hardly accumulate charge, and thereby such a configuration may prevent charging. In addition, a silicon nitride film is disposed between the lower electrode and the upper electrode. The silicon nitride film has a higher relative dielectric constant than that of the silicon oxide film and therefore may have a larger thickness for providing the same capacitance as an insulating film. Accordingly, such a configuration may enhance the dielectric strength.
However, the silicon oxide film has compression stress. In the case of using a silicon oxide film as the membrane that is in contact with the upper electrode on the space side for preventing charging, as in Japanese Patent Laid-Open No. 2008-288813, the vibration film is highly bent due to, for example, buckling. If the bending amount is large, the vibration film may be broken in some cases. In the case of a silicon oxide film, the degree of bending of the vibration film also tends to vary among cells or elements. The variation among the vibration films in bending causes a variation among the elements in sensitivity.